Roman Numeral Parser - FreeCodeCamp #28 Daily Challenge

Roman Numeral Parser: Analysis & Solution

1. Problem Statement

Given a string representing a Roman numeral, return its integer value. Example: “MCMXCIV” → 1994.

Key rule: If a smaller letter comes before a larger one, subtract it; otherwise, add it.

2. Initial Analysis

• What’s required? Convert Roman numerals to integers, following sum/subtract rules.

Test Cases

3. Solution & Code

Strategy:

1. Map each letter to its value.
2. Iterate the string left to right.
3. If the current value is less than the next, subtract; else, add.

// Value mapping
const values: Record<string, number> = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000
}

function parseRoman(s: string): number {
  let total = 0
  for (let i = 0; i < s.length; i++) {
    const current = values[s[i]]
    const next = values[s[i + 1]] ?? 0
    if (current < next) {
      total -= current
    }
    else {
      total += current
    }
  }
  return total
}

Flowchart

flowchart TD
  A["Start"] --> B["Iterate string"]
  B --> C{"Current < Next?"}
  C -- Yes --> D["Subtract current"]
  C -- No --> E["Add current"]
  D --> F["Next letter"]
  E --> F
  F --> G{"End of string?"}
  G -- No --> B
  G -- Yes --> H["Return total"]

4. Complexity

• Time: $O(n)$, single pass through the string.
• Space: $O(1)$, fixed map.

5. Edge Cases

• Empty string → 0
• Invalid input: assume always valid
• Very long numerals: still efficient

6. Reflections

• Simple mapping and conditional logic solve the problem.
• The “look-ahead” pattern is useful in parsing.
• Can be adapted for validation or other numeral systems.

7. Resources

• FreeCodeCamp - Roman Numeral Parser
• Wikipedia - Roman Numerals